The present invention relates, in general, to toolholder systems for machine tools and the like and, more particularly, to a removable tool holding arbor clamped into a socket in the spindle by axial motion of a drawbar.
Removable arbors or toolholders are known for drill presses, milling machines and the like. A typical arbor has a cylindrical shank which mounts into a corresponding socket in the spindle, a central bore therethrough for holding a tool, and a threaded inner end engaged by a drawbar. Tools are typically exchanged in the machine by removing one arbor and replacing it with another in which a different tool is mounted.
Removable arbors are particularly useful in milling and drilling machines such as a Bridgeport milling machine (Bridgeport is a registered trademark of the Bridgeport Machines, Inc.). In such machines, the spindle has an axial bore which opens into an arbor socket, and a drawbar rotatably mounted within the bore. The drawbar has a threaded outer end engaging the threaded inner end of the shank of the tool holding arbor. Rotating the drawbar, either manually or by a drawbar motor, acts to tighten the threaded sections between the drawbar and the arbor, and draws the arbor up into the socket. To release the arbor, the drawbar is rotated in the opposite direction to unscrew the threaded sections from each other.
The outer end of the socket and the outer end of the arbor are flared outward so that the two surfaces engage as the arbor is drawn into the socket. However, the contact surface is not accurate among different arbors to limit exactly how far up the socket the arbor is drawn, especially if the arbors are of different sizes. Varying the placement of different arbors within the socket leads to errors in the finished workpiece.
Also, typical arbors include a relatively long shank which extends almost entirely into the socket for radially locating the arbor, for securing the arbor in the socket, and for receiving and engaging a tool. Replacement of arbors having long shanks requires substantial clearance between the tool or arbor and the workpiece being machined. Removal of the workpiece to replace an arbor is undesirable as it leads to machining errors when the work piece is replaced. As there is a direct relationship between the length of the shank of the arbor and the amount of clearance needed to remove the arbor from the spindle, it is desirable to have an arbor with a short shank.